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Author
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Topic: Sound Rack Wiring
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Barry Floyd
Phenomenal Film Handler
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Posts: 1079
From: Lebanon, Tennessee, USA
Registered: Mar 2000
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posted 07-17-2003 09:20 AM
Our sound rack at the Drive-In will have a SMART MODII B-DIT processor for the film sound, a CD player,Tape Deck,and microphones running thru a small rack mount mixer, and an FM transmitter for the final output to the cars in the field.
My question is, the non-sync "inputs" on the back of the SMART unit are screw post terminals, whereas the output from the mixer is two RCA jacks. How do I transition between these two and still get a clean signal?
Also, the "outputs" from the SMART processor to the transmitter are also screw post terminals, but the inputs to the transmitter are two XLR jacks. How will I connect these two?
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Bruce Hansen
Jedi Master Film Handler
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Posts: 847
From: Stone Mountain, GA, USA
Registered: Dec 1999
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posted 07-17-2003 03:48 PM
In the days when balanced inputs and outputs were done with transformers (inside the equipment), you would connect pins 1 and 3 as the shield and use pin 2 as the hot; but today most equipment uses IC balancing. On the output of the device that uses XLR's: if you connect pin 3 to pin 1 you are shorting the output of the IC feeding pin 3 to ground. This is a no no, and can cause problems such as distortion, and cross talk. I like to use pin 1 as the shield, pin 2 as the hot, and connect a 1K 1/8 watt resistor batween pins 1 and 3. If you are doing this as a perment install, on the output of the device using XLR's, use pin 1 as the shield, and pin 2 as the hot, do not connect pin 3. On the input to the device using XLR's, you can connect pins 1 and 3 together, and use that as the shield, and use pin 2 as the hot. Some people will also use pin 3 as the shield, and pin 2 as the hot on the input to a device using XLR's. Some equipment manuals, today, have diagrams that show you how to do all this.
Another problem is that RCA connectors normanly have about -10 db on them, XLR's normanly have +4 db on them. There are units made that will convert properly between XLR's and RCA's, both balancing, and unbalancing properly, and changing the levels.
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Oscar Neundorfer
Master Film Handler
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Posts: 275
From: Senoia, GA
Registered: May 2000
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posted 07-18-2003 07:04 AM
Barry,
For the non-sync inputs, obtain a standard dual (stereo style)rca plug patch cable and cut the plugs off one end. This will leave rca plugs on one end of the cable. On the other end where you cut off the plugs, separate the two cables (these are usually joined like zip cord), and strip the wires outer insulation back about an inch and a half. You will see the outer shield wire wrapped around the inner insulated wire. Unwrap the shield wire and twist the individual strands together. This will become the ground connection. Now strip back the inner insulation to expose the inner conductor. This wire will become the left or right non-sync input connection. You may connect these wires directly under the screw terminals, or you may choose to use spade lugs and solder the wires to the spade lugs which will then go under the screw teminals. Either way is acceptable although the spade lugs may be a little more robust. The wires in these cables are usually rather small and somewhat fragile. It may be a good idea to tie wrap the cable at some point to relieve any tension on the connections themselves.
Regarding the outputs to the transmitter, you will just have to make these cables yourself. Get the proper xlr connectors and some 2 conductor plus shield cable. Connect the shield and one of the conductors to pin 3 of the xlr, and then jumper pin 1 to pin 3. You will have pins 1 and 3 and the shield and one conductor all tied together. Now connect the other conductor to pin 2 of the xlr.
PLEASE NOTE: MOST manufacturers use pin 2 of the xlr as the hot or non-inverting input. If your transmitter uses pin 3 as hot, you may need to reverse the pin designations in the above instructions. If it is a truly balanced input, then it probably will not matter anyway whether you use pin 2 or pin 3 as the input. Pin1 is always ground.
At the processor end, cut off the shield (no connection) and connect the conductor which went to pin 3 of the xlr to the ground terminal. Connect the conductor which went to pin 2 of the xlr to the hot output terminal. You will need one cable for each channel (left and right).
This should normally work fine. However, there may be some reason the transmitter needs to have a truly balanced input (the SMART unit's output is unbalanced). But usually this arrangement works OK.
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